The present invention relates to fluid flow regulation, and also to method and apparatus for regulating the metering of liquid fuel as atmospheric pressure changes to thereby produce an air-fuel mixture having a substantially constant air-to-fuel ratio.
U.S. Pat. No. 3,778,038, issued Dec. 11, 1973, describes a method and apparatus for producing a uniform combustible mixture of air and minute liquid fuel droplets for delivery to an internal combustion engine. The apparatus includes an intake air zone connected to a variable area constricted zone for constricting the flow of air to increase the velocity thereof to sonic. Liquid fuel is introduced into the air stream at or above the constricted zone to divide and uniformly entrain fuel as droplets in the air flowing through the constricted zone. Walls downstream of the constricted zone are arranged to provide an increasing cross-sectional area for efficiently converting a substantial portion of the kinetic energy of the high velocity air and fuel to static pressure. Through such conversion it is possible to maintain sonic velocity air flow through the constricted zone over substantially the entire operating range of the engine.
The above U.S. patent further explains the well known phenomena that under sonic conditions, the pressure of the air at the constricted zone is approximately 53% of atmospheric pressure. Under sonic conditions and when the atmospheric pressure remains constant, it is possible to provide an air-liquid fuel mixture having a substantially constant air-to-fuel ratio by simply metering the amount of fuel delivered into the air stream in direct proportion to the area of the constricted zone. However, when atmospheric pressure varies, possibly due to altitude changes, the mass flow rate of air passing through the apparatus also varies. When this occurs it is necessary to adjust the amount of fuel introduced into the air stream in order to maintain a substantially constant air-to-fuel ratio. For example, when atmospheric pressure decreases, the air passing through the device has less mass density and less fuel is required to produce a mixture having the same air-to-fuel ratio as before the atmospheric change. A fuel metering system which relies solely upon the area of the constricted zone or the volume of air passing therethrough does not correct for such atmospheric fluctuations, and the air-to-fuel ratio varies depending upon varying atmospheric conditions.
In order to accurately compensate for atmospheric pressure changes it is necessary that variations in the pressure differential across the fuel metering valve be accompanied by directly proportional variations in the mass flow rate of fuel through the valve. While laminar-flow metering valves meet such specifications, the mass flow rate of fuel through these valves is inversely dependent upon the kinematic viscosity of the liquid. For example, over a temperature range of 20.degree. to 100.degree. F., the kinematic viscosity of gasoline changes approximately by a factor of two which results in a system highly sensitive to fuel temperature. It is extremely difficult, if not impossible, or excessively expensive to compensate for such temperature dependence in producing a laminar-flow valve. Accordingly, even though the mass flow rate of fluid through a laminar-flow valve varies directly with the pressure differential across the valve, its inverse dependence on kinematic viscosity makes it totally unacceptable from the standpoint of corrective fuel metering for atmospheric pressure fluctuations. On the other hand, while turbulent-flow valves, such as needle valves, are not sensitive to the kinematic viscosity of liquids, the mass flow rate through such a valve does not vary directly with the pressure differential across the valve.